U.S. patent application number 13/509014 was filed with the patent office on 2012-10-18 for method for the treatment of water and wastewater.
Invention is credited to Hans F. Larsson.
Application Number | 20120261357 13/509014 |
Document ID | / |
Family ID | 42537765 |
Filed Date | 2012-10-18 |
United States Patent
Application |
20120261357 |
Kind Code |
A1 |
Larsson; Hans F. |
October 18, 2012 |
METHOD FOR THE TREATMENT OF WATER AND WASTEWATER
Abstract
A method and a plant for the treatment of water or wastewater,
having impurities therein by filtration through two granular media
filter stages (1, 2) of the moving bed type operated in series,
comprising: feeding of said water/wastewater as a first influent to
first stage granular media filters; filtration of said first
influent in said first stage granular media filters to produce a
first effluent; feeding said first effluent as a second influent to
second stage granular media filters; and filtration of said second
influent in said second stage granular media filters to produce a
second effluent; wherein said second stage granular media filters
(2) are operated with intermittent washing of the granular filter
media.
Inventors: |
Larsson; Hans F.;
(Stockholm, SE) |
Family ID: |
42537765 |
Appl. No.: |
13/509014 |
Filed: |
November 11, 2009 |
PCT Filed: |
November 11, 2009 |
PCT NO: |
PCT/EP2009/065011 |
371 Date: |
June 28, 2012 |
Current U.S.
Class: |
210/786 ;
210/269 |
Current CPC
Class: |
B01D 24/30 20130101;
B01D 24/46 20130101; B01D 24/007 20130101; B01D 24/4689
20130101 |
Class at
Publication: |
210/786 ;
210/269 |
International
Class: |
B01D 24/28 20060101
B01D024/28; B01D 24/48 20060101 B01D024/48; C02F 1/00 20060101
C02F001/00; B01D 24/46 20060101 B01D024/46 |
Claims
1. A method for the treatment of water or wastewater, having
impurities therein, by filtration through two granular media filter
stages of the moving bed type operated in series, comprising:
feeding of said water/wastewater as a first influent to first stage
granular media filters; filtration of said first influent in said
first stage granular media filters to produce a first effluent;
feeding said first effluent as a second influent to second stage
granular media filters; and filtration of said second influent in
said second stage granular media filters to produce a second
effluent; characterized by intermittent washing of the granular
filter media of said second stage granular media filters.
2. The method according to claim 1, characterized in that the
second stage granular media filters are operated with continuous
filtration.
3. The method according to claim 1, characterized in that the
second stage granular media filters are operated with intermittent
filtration and that water used for washing of the filter media is
replaced with suitably clean water.
4. The method according to claim 3, characterized in that the
replacement water consists of water produced as second
effluent.
5. The method according to claim 1, characterized in that the first
stage granular media filters are operated with continuous
filtration and continuous granular media washing.
6. The method according to claim 1, characterized in that the first
stage granular media filters are operated with continuous
filtration but with intermittent granular media washing.
7. The method according to claim 1, characterized in that the first
stage granular media filters are operated with intermittent
filtration and intermittent washing and that water used for washing
of the filter media is replaced with suitably clean water.
8. The method according to claim 7, characterized in that the
replacement water consists of water produced as first and/or second
effluent.
9. The method according to claim 1, characterized in that the
granular filter media in said granular media filters is washed by
removing granular filter media from the bottom part of the filter
bed, transporting it to a media washer, washing it and returning it
to the top of the granular filter media bed, while a reject
consisting of wash water containing pollutants is produced.
10. The method according to claim 9, characterized in that granular
filter media is transported from the bottom part of the filter bed
with an air lift pump to a media washer, that washed filter media
is returned to the top of the granular filter media bed and that
reject is discharged through a reject pipe.
11. The method according to claim 10, characterized in that a valve
means in the reject pipe is kept open only during the washing of
filter media.
12. The method according to claim 10, characterized in that a valve
means in the reject pipe is opened a suitable time before starting
the air lift pump and is closed at a suitable time after the air
lift pump is stopped.
13. The method according to claim 11, characterized in that a
continuous fractional flow of water through the reject pipe is
maintained between washings.
14. A plant for the treatment of water or wastewater, having
impurities therein, for performing a method according to claim 1,
comprising at least one first stage free-standing granular media
filter module of the moving bed type or at least one filter cell
(40a, b, c) comprising at least one first stage granular media
filter module (42a, b, c, d) of the moving bed type and at least
one second stage free-standing granular media filter module of the
moving bed type or at least one filter cell (46a, b, c) comprising
at least one second stage granular filter media filter module (48a,
b, c, d) of the moving bed type, operated in series, the effluent
from first stage granular media filter module (s) or filter cell
(s) being the influent of second stage granular media filter module
(s) or filter cell (s), characterized in that the second stage
granular media filter module (s) (2; 48a, b, c, d) is (are)
arranged to be intermittently washed and is (are) provided with
controlled valve means (4; 33; 59) for stopping reject flow between
washings.
15. A plant according to claim 14, characterized in that the first
stage granular media filter modules (1; 42a, b, c, d) are arranged
to be intermittently washed and are provided with controlled valve
means (60; 33; 58) for stopping reject flow between washings.
16. The plant according to claim 14, characterized in that the
valve means (33) comprises a by-pass arrangement (34, 35) which
admits a continuous fractional flow of water between washings.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the treatment of
water/wastewater, and more particularly, to an improved method for
removing impurities/pollutants, for example particles,
precipitates, metals, emulsions, algae, bacteria, viruses, Protozoa
and their oocysts as well as other microorganisms and related
matter from water/wastewater according to the preamble of claim
1.
BACKGROUND OF THE INVENTION
[0002] Water normally needs to be purified before it is used by
municipalities and/or industries, and the wastewater produced by
municipalities and/or industries needs to be treated before it is
reused and/or discharged. The quality demands for the treated
water/wastewater have got stricter over time due to stricter
drinking water and environmental regulations. The need to use new
sources for drinking water and process water like seawater and/or
polluted surface water and/or treated wastewater further creates a
need for advanced and reliable treatment methods and/or systems to
produce acceptable water.
[0003] New methods and systems are needed both for complete stand
alone systems as well as for specific treatment steps in a chain of
treatment steps, for example pre-treatment before membranes for
seawater desalination.
[0004] Another such system may be a water treatment system where
drinking water or industrial process water is produced from
seawater and/or surface water and another system may be a
wastewater treatment system, wherein the wastewater needs to be
treated so that it can be discharged or reused in industry or in
municipalities or for irrigation, as a part source of drinking
water and similar purposes. A further example is the treatment of
lake and/or river water and/or ground water to produce drinking
water and/or process water.
[0005] Such methods and systems should be simple, reliable and
produce treated water of a very high quality. Such treatment
systems and/or steps should further be energy efficient and use as
little chemicals as possible and produce as little reject and/or
polluting by-products of the treatment as possible.
[0006] In order for such treated water as mentioned in the examples
to be useful, particles, dissolved substances, algae, bacteria,
viruses, Protozoa, organic substances, phosphorus and other
nutrients, arsenic, metals and other pollutants may in many cases
need to be removed to a very high degree from the water/wastewater.
Moreover microorganisms, such as
[0007] Cryptosporidium and Giardia and their oocysts and/or cysts,
may need to be removed from the water/wastewater. Many systems have
one or more steps that convert dissolved or colloidal matter to
particles that can be separated by solids/liquid separation
techniques. In such a purification process and/or as a treatment
step in such a purification process, the water/wastewater may be
subjected to precipitation and/or flocculation treatment. Dissolved
substances such as humic substances, metals, nutrients e.g.
phosphorus and/or poisonous substances like arsenic and its
compounds, fluorides and/or pesticides are converted to solid
particles and/or absorbed or adsorbed on solid particles. Such
particles may be of colloidal size and/or created by precipitation
and may need to be flocculated in order to create bigger particles.
Colloidal matter and other fine particles present in the
water/wastewater may also need to be subjected to precipitation
and/or flocculation treatment in order to create bigger
particles.
[0008] In this regard, conventional chemical treatment can include
chemical injection and flash mixing/precipitation followed by one
or more flocculation tanks in which the water/wastewater is
agitated with stirrers or agitators in order to create bigger
particles, flocs after which it passes through one or more
sedimentation basins for separation of particles and/or flocs. One
of the disadvantages of conventional chemical treatment processes
is the large area and/or volume required for the flocculation tanks
and sedimentation basins. A further disadvantage of conventional
chemical treatment techniques is the long residence time for the
water/wastewater in the flocculation tank as well as in the
sedimentation basin.
[0009] The use of chemicals addition, flocculation tanks and
sedimentation basins alone in the chemical treatment process does
not typically result in a high enough water purity for many
applications. While membrane filtration with a suitably tight
membrane can be used to attain a higher level of purification, such
membrane filters are expensive and have other disadvantages. On the
other hand, a granular media filter, for example a sand filter, can
be added in the treatment chain to increase the purity of the water
being treated. The granular filter media in such granular bed
filters must be cleaned. In some such filters the granular filter
media is cleaned by being subjected to back-washing at intervals
and/or when the pressure drop over the filter bed has reached a
predetermined level. This means that the filtration has to be shut
off while the granular bed filter is backwashed. Further, during a
period of time after backwashing such granular bed filters produce
a first filtrate which is of low quality and has to be discharged
when high quality water is to be produced. The reason for the low
quality of the first filtrate is that after back-washing the
granular filter bed is clean and free from separated solids.
However, such separated solids assist the separation in the filter
bed and therefore their absence leads to low filtrate quality.
Great savings may be obtained if a continuously operated granular
bed filter of the moving bed type is used. The most common type of
granular filter media used is filter sand. Such filters are
described in U.S. Pat. No. 6,426,005 B1, U.S. Pat. No. 4,126,546,
U.S. Pat. No. 4,197,201 and U.S. Pat. No. 4,246,102, as well as in
U.S. Pat. No. 5,843,308. The filters described in these patent
documents are of the moving filter bed type.
[0010] In such a filter chemicals may be added to the influent to
the filter and precipitation, flocculation and separation can all
take place in the filter bed as described in U.S. Pat. No.
4,246,102 and in U.S. Pat. No. 6,426,005 B1. A further advantage is
that the continuous granular bed filter will not have to be taken
out of operation for backwashing and thus there will not be a need
for extra capacity to take care of the flow that should have been
treated in the conventional filters in a conventional plant being
backwashed. There will be no first filtrate in a continuous sand
filter since the continuous washing is arranged so that part of the
granular media is continuously taken out of the filter bed, washed
and returned to the filter bed, so that a steady state is reached
where a suitable amount of separated particles is always left in
the granular filter bed. Continuous sand filters are well known
today and there are many plants having continuous sand filters
working as a purification step in water treatment systems for
municipal and industrial water and wastewater.
[0011] The continuous sand filters generally work well but as
mentioned above there are applications were really high purity of
the treated water is required and where e.g. membrane treatment
would be required. Membrane plants are however expensive, sensitive
and costly to run due to high energy demand and the necessity to
exchange the membranes at regular intervals. Furthermore the
cleaning of the membranes normally produces large quantities of
reject water and in many cases poisonous chemicals are needed for
the cleaning of the membranes. Therefore, there is a need for more
economical treatment systems that are simpler, more robust and more
energy efficient and produce less reject or wash water but still
produce a very high quality filtrate as the effluent.
[0012] An example of such a systems is e.g. disclosed in U.S. Pat.
No. 6,426,005 B1 relating to a method and system for treating
water/wastewater including two serial, continuously operating
granular media filters of the moving bed type with continuous
washing of the granular bed media, e.g. moving bed sand filters.
The type of system disclosed in U.S. Pat. No. 6,426,005 B1 can
either be used as a stand alone system for treatment of water
and/or wastewater or be used as a part of a treatment system
consisting of a number of steps.
[0013] The treatment system according to U.S. Pat. No. 6,426,005 B1
thus comprises two serial continuously operating granular media
filters of the moving bed type that may have different size of
filter media, e.g. filter sand. In addition, chemicals for
coagulation/flocculation may be added and the liquid to be treated
may be subjected to a disinfecting treatment and/or a mechanical,
biological and/or chemical treatment. The water/wastewater to be
treated is introduced as an influent into said filters. The
water/wastewater is treated within said filters such that treated,
processed water/wastewater or effluent is produced and the
impurities separated in the bed in the first granular media filter
are discharged from the first granular media filter as a first
reject. The effluent from the first granular media filter is
further filtered in the second continuously operating granular
media filter such that the effluent from the second granular media
filter is the filtrate and the impurities separated in the bed in
the second granular media filter are discharged from the second
granular media filter as a second reject. In order to further
concentrate the pollutants in the first and second rejects, the
first and second rejects being discharged from the first and second
continuously operated granular media filters, respectively, are
introduced into a separate treatment apparatus. In such a treatment
apparatus, the rejects containing pollutants separated from the
water/wastewater being treated in the first and second serial
granular media filters are subjected to a renewed treatment and/or
separate treatment that eventually results in purified water that
meets quality standards and a sludge that can be dewatered and/or
processed, as its end products.
[0014] However, although this system works very well, and high
quality water can be produced with it, there is still a need to
produce even cleaner treated water or effluent. For example such a
system cannot from many sources of seawater and/or water from
common sweet water sources for drinking water/process water
consistently produce water having a purity measured as SDI (Silt
Density Index) 4 or lower. Considering that the usual requirement
for a pre-treatment step upstream of a reverse osmosis membrane
(R/O) is that the SDI value should at all times be equal to or
lower than SDI=4 it is of the utmost importance to be able to
fulfill this requirement in an economical and efficient way.
[0015] Pure water is a limited resource in the world, while at the
same time sea water is an immense resource for drinking water. Both
these factors have influenced companies and inventors to develop
methods and means for water purification and techniques for
desalination of sea water. One technique for doing this is reverse
osmosis.
[0016] Reverse osmosis which uses membranes that are both expensive
and very sensitive is a realistic technique for desalination of sea
water, but it is essential that the influent to a reverse osmosis
plant is substantially free from particulate matter and other
pollutants, or expressed in other terms, has a silt density index
SDI, which does not exceed SDI 4, and preferably is even lower. It
is not possible to consistently reach this high purity level when
treating feed water with a substantial variation in the level of
pollutants with existing techniques using granular media filters,
and as mentioned above with the advanced system according to U.S.
Pat. No. 6,426,005 B1, only under favourable circumstances is it
possible to obtain an SDI of around 5 or lower.
[0017] In the treatment of municipal wastewater a far reaching
reduction of nutrients is required in order to prevent
eutrophication of rivers and lakes. Some municipalities demand a
level of total phosphorus of 0.02 milligrams per litre or lower.
This is normally not possible to consistently achieve by the use of
conventional granular bed filters with chemical treatment and even
with a system such as that described in U.S. Pat. No. 6,426,005 B1.
Consequently improved systems that are reliable and cost efficient
and that can achieve this effluent quality are highly
desirable.
[0018] In other applications bacteria, viruses and/or
microorganisms like Giardia and Cryptosporidium and their oocysts
need to be removed to the highest degree. Also here improved
systems are highly desirable.
[0019] Arsenic in drinking water sources presents another example
where high efficiency separation in a simple, robust, and reliable
system is much needed.
[0020] Ground water may contain particles, metals and/or dissolved
substances that may need to be removed to a high degree.
[0021] The examples just represent a few cases of many such
applications where a very high degree of purification is needed and
where the method and system should be cost effective, simple to
operate and reliable.
SUMMARY OF THE INVENTION
[0022] One object of the present invention is to provide an
improved method for the treatment of water and/or wastewater which
is suitable for all applications where particles and/or other
pollutants and/or emulsions are to be separated to a very high
degree in order to produce a very clean effluent.
[0023] Another object of the invention is to provide an improved
method for the treatment of seawater and/or other salt containing
water that makes it possible to consistently produce water which is
sufficiently pure (SDI consistently below 4) to be treated in a
desalination plant working with reverse osmosis membranes, and at
the same time provide a robust system being able to cope with
comparatively low quality influents with characteristics that may
vary strongly due to circumstances, e.g. storms leading to sharply
increased silt levels, algal growth, construction and/or shipping
activity near the seawater intake.
[0024] Another object of the invention is to provide an improved
method for production from surface water, ground water and/or
wastewater of drinking water and/or industrial process water that
must be of a high quality like e.g. boiler feed water. In such
treatment particles, humic substances, microorganisms, viruses,
arsenic, metals, fluorides, pesticides and a number of other
substances may have to be removed.
[0025] Another object of the invention is to provide an improved
method for removing particles from water that has been subjected to
precipitation down to very low levels of pollutants, e.g. 0.02 mg/l
or lower for total phosphorus. Other examples where a very high
separation efficiency after precipitation can be obtained is for
the removal of arsenic, metals and/or fluorides from water as well
as the removal of
[0026] Giardia and Cryptosporidium and their cysts and oocysts as
well as other microorganisms. The mentioned applications are just
examples and the method and system of the invention is suitable in
all cases where a very clean water is to be produced from water
and/or wastewater containing impurities and where the impurities
are in particle form and/or can be transformed into particle form
and/or may be adsorbed or absorbed on activated granular carbon
and/or other adsorbents/absorbents and/or granular media coated
with adsorbents and/or absorbents and/or reacted with the help of
granular catalyst particles and/or granular media coated with
catalysts and/or other reactants.
[0027] A further object of the present invention is to provide a
system or plant for the treatment of water which consumes less
energy and produces less reject to be taken care of compared with
prior art processes.
[0028] Another object of the invention is to provide a system or
plant that uses less chemicals to obtain the desired high purity of
the effluent.
[0029] These and other objects of the invention will be achieved
with the method according to claim 1 with the features defined in
the characterizing part, and the plant according to claim 14.
Developments and preferred embodiments of the invention are defined
in the sub claims.
[0030] The improved method according to the invention for the
treatment of water or wastewater, having impurities therein, uses
two granular media filter stages of the moving bed type, operated
in series. Intermittent washing of the granular filter media in the
second or downstream granular media filter stage according to the
invention, very surprisingly produces a substantial improvement of
the effluent when used on the same influent and the under the same
conditions as for the system according to U.S. Pat. No. 6,425,005
B1. For instance, when treating sea water as a pre-treatment for
reverse osmosis the effluent resulting from the method and system
according to the invention had a SDI of 4 or lower with chemical
addition, while the prior art system operated on the same type of
water and under equal conditions but with continuous washing of the
second stage granular filter media bed produced an effluent with an
SDI of approximately 5.
[0031] Even more surprising, after optimising the filtration system
and/or plant according to the invention and using bigger filters a
treated water of around SDI 3 was obtained without chemical
addition. Since the cost of chemicals constitutes a big part of the
cost of operation this leads to a major cost saving. In addition to
the improved effluent quality and the savings related to the cost
of chemicals the intermittent washing operation in the second stage
granular media filter led to a much lower energy consumption and a
much lower production of reject water.
[0032] According to one embodiment of the invention, granular media
filters in the second stage are operated with continuous
filtration.
[0033] According to another embodiment of the invention, the second
stage granular media filters are operated with intermittent
filtration and intermittent washing, i.e. the filtration is stopped
during the intermittent washing, and water used for washing of the
filter media is replaced with suitably clean water.
[0034] According to a further embodiment of the invention the first
stage granular media filters are operated with continuous
filtration and continuous granular media washing.
[0035] According to a further embodiment of the invention the first
stage granular media filters are operated with continuous
filtration but with intermittent granular media washing.
[0036] According to a further embodiment of the invention the first
stage granular media filters are operated with intermittent
filtration and intermittent washing, i.e. the filtration is stopped
during the intermittent washing, and water used for washing of the
filter media is replaced with suitably clean water.
[0037] According to a further embodiment of the invention, during
intermittent filtration water used for washing of the filter media
is replaced with suitably clean water, preferably effluent from a
parallel filter being in filtration mode.
[0038] According to a further embodiment of the invention, each of
said first and second stage granular media filters are washed by
removing granular filter media from the bottom part of the filter
bed, transporting it to a media washer, washing it and returning it
to the top of the filter bed, while a reject consisting of wash
water and pollutants is produced.
[0039] According to a further embodiment of the invention granular
filter media is transported from the bottom part of the filter bed
with an air lift pump to a media washer, the washed filter media is
returned to the top of the filter bed and the reject produced
consisting of wash water and pollutants is discharged through a
reject pipe.
[0040] According to a further embodiment of the invention a valve
in the reject pipe is kept open only during the washing of filter
media.
[0041] According to a further embodiment of the invention the valve
in the reject pipe is opened at a suitable time before starting the
air lift pump and is closed at a suitable time after the air lift
pump is stopped.
[0042] According to a further embodiment of the invention a
continuous fractional flow of water through the reject pipe is
maintained between washings.
[0043] According to a further embodiment of the invention, coarser
granular media is used in said first stage granular media filters
in relation to in said second stage granular media filters.
Further, different densities of the granular media can be used in
the first and second stage granular media filters,
respectively.
[0044] According to a further embodiment of the invention, said
first stage granular media filters are arranged with their effluent
pipes at a higher level above ground in relation to the influent
pipes of said second stage granular media filters.
[0045] According to a further embodiment of the invention a shorter
filter bed is used in the second stage filters.
[0046] According to a further embodiment of the invention the
rejects from the first and second filter stages are either
discharged or subjected to further treatment and/or disinfection in
order to concentrate and/or make harmless the rejects, or they are
returned to an earlier step in the treatment chain. The reject from
the second filter stage may as an alternative be returned to the
influent of the first stage granular media filters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The invention will now be described more in detail in the
following description with reference to the accompanying drawings,
of which
[0048] FIG. 1 schematically illustrates a two stage filtering
system with granular media filter modules of the moving bed type
for performing the method according to the invention, where each of
the first and second stages consist either of any number of
free-standing granular media filter modules operated in parallel
(in case of more than one filter) or consist of a number of
granular media filter modules arranged in filter cells, in which
case a filter plant will comprise an arbitrary number of cells in
each filter stage being arranged to work in parallel with each
other (in case of more than one cell).
[0049] FIG. 2 schematically illustrates a system for treatment of
sea water or surface water according to the invention and with
optional chemical treatment
[0050] FIG. 3 schematically illustrates the discharge of reject
water from the granular media washer with a reject valve and a
bypass on the reject pipe.
[0051] FIG. 4a schematically illustrates part of a filtration plant
according to the invention.
[0052] FIG. 4b is a cross-sectional view taken along the line
IVb-IVb in FIG. 4a.
[0053] FIG. 4c is a cross-sectional view taken along the line
IVc-IVc in FIG. 4a.
[0054] FIG. 5 illustrates a prior art water/wastewater treatment
apparatus including two serial, continuously operated sand filters
with continuous filtration and continuous washing of the granular
filter bed media.
DETAILED DESCRIPTION OF THE INVENTION
[0055] The prior art filter system according to U.S. Pat. No.
6,426,005 B1 is schematically illustrated in FIG. 5 of the
drawings. The working principle is exhaustively explained in the
specification of said patent, which is incorporated herein by
reference, in its entirety.
[0056] In the prior art process described in U.S. Pat. No.
6,426,005 B1, a first, or upstream filter A works with continuous
filtration of water/wastewater fed upwards through the granular
filter media, while the bed of granular filter media moves
downwards. A part of the dirty granular filter media is transported
with an air lift pump up to the top of the filter and is washed and
returned to the top of the filter bed. In the prior art device the
second or downstream filter B works according to the same principle
i.e. continuous filtration and continuous, uninterrupted washing of
the filter media.
[0057] FIG. 1 illustrates a filter system for performing the method
according to the invention. The system comprises a first 1 and a
second 2 stage granular filter media filter of the moving bed type,
corresponding to the filters disclosed in U.S. Pat. No. 6,246,005
B1.
[0058] According to the invention, the second stage granular media
filter 2 is operated with intermittent washing of the granular
filter media. Granular filter media is transported by an air lift
pump 31 from the bottom part of the granular filter media bed to a
media washer 5. The reject pipe 3 is provided with a valve 4 so
that the flow of wash water through the media washer can be turned
off when there is no washing and consequently no need for wash
water. It is advantageous to arrange the control of this valve so
that it opens a suitable time before the air lift pump is started
and closes at a suitable time after the air lift pump is stopped.
This is to assure that there is a flow of wash water already when
the first granular media particles enter the media washer when the
air lift pump is started, and so that all granular media particles
in the media washer when the air lift pump is stopped will be
washed. The air lift pump is provided with a control, shown as a
controlled valve 6 in a pipe for compressed air 7 for turning the
air flow on and off i.e. starting and stopping the pumping action
of the airlift pump 31, and thus together with the valve 4
controlling the intermittent washing of the granular filter
media.
[0059] By controlling the reject valve 4 and the air lift pump 31,
for instance allowing two washing periods of 30 minutes each per 24
hours the consumption of pressurized air, and thus energy
consumption for washing of granular filter media, in the second
stage granular media filter 2 will be reduced by more than 95% and
the reject volume will be reduced by a factor 20 making it much
easier to take care of and/or treat.
[0060] In case the incoming water to the first stage granular media
filter 1 contains a low amount of impurities then it may be
advantageous to operate also the first stage granular media filter
1 with intermittent washing of the granular filter media.
[0061] The reject consisting of wash water and pollutants,
discharged through reject pipe 8 from the first stage granular
media filter and through reject pipe 9 from the second stage
granular media filter resulting from the two stage filtration
according to the invention may be taken care of according to any of
the methods described in U.S. Pat. No. 6,426,005 B1, or when the
two stage filtration is part of a larger treatment system it may be
returned to an upstream treatment step or alternatively the reject
may be discharged directly e.g. into the sea or a lake in case its
composition allows this. The reject from the second stage granular
media filter can also be returned to the influent to the first
stage granular media filter, either directly upstream of the first
stage granular media filter or upstream of or together with
injection of chemicals. The process and the operation of the two
granular media filter stages is controlled from a control unit
10.
[0062] FIG. 2 shows a large scale two stage granular media filter
system arranged for a bigger plant or as part of a bigger plant
with optional chemicals addition. In the illustrated example, lake,
river or sea water 11 is taken into a plant through an intake
screen 12, through a feed pipe 13. Chemicals can be introduced into
the feed pipe 13 from a dosing apparatus 14 directly or upstream of
a mixing device, such as static mixer 15 or a dynamic mixer, before
the first granular media filter stage 16. When addition of
chemicals is discussed with reference to static mixers in the
following description, this is one example. Dynamic mixers and
direct addition of chemicals can be used instead.
[0063] In case chemicals are added precipitation, flocculation and
separation can be carried out in the first granular media filter
stage 16 and the treated effluent from the first granular media
filter stage 16 is further polished by filtration in a second
granular media filter stage 17, so that remaining particles and/or
flocs can be separated in order to produce a very clean effluent
leaving the second granular media filter stage 17 through the pipe
18. In some cases it is advantageous to add chemicals only to the
first granular media filter stage 16, e.g. via the static mixer 15,
to both the first granular media filter stage 16 e.g. via the
static mixer 15 and the second granular media filter stage 17, e.g.
via a static mixer 19, only to the second granular media filter
stage e.g. via the static mixer 19, or no chemical addition at all.
In many cases where a system according to the prior art of U.S.
Pat. No. 6,246,005 B1 requires the addition of chemicals, the
system according to the invention can produce cleaner effluent
without any addition of chemicals which leads to a great cost
saving. Reject in the form of wash water containing pollutants
separated from the granular filter media leaves the first granular
media filter stage through the pipe 20, and from the second
granular media filter stage through the pipe 21. These reject
streams can be taken care of in different ways, which will be
discussed further below.
[0064] FIG. 3 shows a media washer 30 used according to the
invention for washing granular filter media taken from the bottom
part of the granular filter media bed and transported with an air
lift pump 31, the top of which is shown in FIG. 3. A reject pipe 32
transporting the reject from the media washer is also shown.
[0065] A valve 33, also discussed in connection with FIG. 1 with
the reference numeral 4, is arranged in the reject pipe 32.
[0066] When producing extremely clean water every potential source
of pollution must be eliminated. One such potential source of
pollutants when operating a granular media filter of the moving bed
type with intermittent filter media washing is the possibility
that, between the washing periods, a small flow of water passes up
through the air lift pump tube without being sufficiently filtered
in the granular filter media bed and escapes into the effluent by
moving downward through the filter media washer.
[0067] According to the invention such pollution is prevented by
allowing a fractional flow of water to pass up through the media
washer and out through the reject pipe 32 and carrying with it any
pollutants that may have escaped from the air lift pump. This can
be achieved with a bypass conduit 34 arranged around the valve 33.
This by-pass is also provided with a shut-off valve 35. As an
alternative, the valve 33 may be constituted by a valve means which
does not close completely or which, when washing is not performed,
may be controlled to let a fractional flow of water pass through
the valve.
[0068] FIG. 4a shows a schematic top view, FIG. 4b shows a
cross-sectional view taken along the line IVb-IVb in FIG. 4a, and
FIG. 4c shows a cross-sectional view taken along the line IVc-IVc
in FIG. 4b, of a plant comprising first stage granular media
filters of moving bed type and second stage granular media filters
of moving bed type arranged in cells each containing four granular
media filter modules. The layout shown in FIG. 4a has been chosen
only for the ease of understanding, and it is obvious that any
number of cells can be used containing any number of granular media
filter modules.
[0069] In a concrete structure three first stage filter cells 40a,
b, c, each containing four granular media filter modules 41a, b, c,
d are arranged. A first influent, which could be sea water, lake
water, river water, ground water or water from a preceding process,
enters the plant through a feed pipe 42 into a trough 43
communicating with each of the filter modules through first feed
pipes 44a, b, c. A first effluent from the filter cells 40a, b, c
enters a second trough 45, which is connected to the second stage
filter cells 46a, b, c through second feed pipes 47a, b, c, for
supplying this first effluent from the first stage filter cells as
a second influent to the second stage filter cells containing four
second stage granular media filter modules 48a, b, c, d. A second
effluent from these second stage filter cells enters into a third
trough 49 and leaves the plant through the outgoing pipe 50.
[0070] The second feed pipes 47a, b, c are provided with valves
51a, b, c for an optional closing of the supply of second influent
into the second stage filter modules, i.e. intermittent filtration
in the second stage filter cells.
[0071] When producing extremely clean water (SDI around 3 or lower)
every possible source of pollution of the effluent has to be
avoided. In order to avoid any risk that particles enter into the
effluent from the second stage granular media filter cells,
filtration may be turned off during washing and wash water used for
washing the granular media can be replaced with suitably clean
water. This can be done from an external source, such as through a
pipe 52. Alternatively, the wash water can be replaced with
effluent from another filter cell being in filtering mode. This
could be accomplished using a tube 53 connecting the effluent
volumes of the cells between two adjacent filter cells, or with the
use of pump means 54a, b. The tube is preferably provided with
controlled valve means 55.
[0072] The reason for this arrangement is to prevent that particles
that have been captured in the granular filter media bed are
dislodged by the movement of the media particles during the washing
and escape into the effluent. Even if the effect is small there is
an obvious need to eliminate it when producing an extremely clean
effluent.
[0073] For sake of clarity of FIGS. 4a, b and c a tube 53 is shown
only between the adjacent cells 46b and 46c and pump means 54a, b
only between the adjacent cells 46a and 46b. The means chosen for
the communication between cells is preferably mounted between all
adjacent cells working in parallel. Further, reject pipes have been
left out in FIG. 4a. In FIGS. 4b, c is schematically shown reject
pipes 56 and 57 from the first stage granular media filter modules
and second stage granular media filter modules, respectively. The
reject pipe in each cell is connected to all filter modules in the
cell, which is illustrated with pipes 57a, 57b in FIG. 4c. As
discussed above, the reject pipes 56, 57 are provided with valve
means 58, 59, which are controlled from a control unit, which
however is not illustrated in the Figs. for the sake of
clarity.
[0074] Consequently, depending on the circumstances, the filtration
may continue during the washing, or it can be stopped, that is, in
that case both the filtration and the washing are performed
intermittently.
[0075] In the embodiment illustrated in FIGS. 4a-c, the plant is
laid out for continuous filtration and continuous washing in the
first stage granular media filters. However, it is also possible to
operate said first stage granular media filters with continuous
filtration and intermittent washing, and with intermittent
filtration, i.e. stopping the filtration during the intermittent
washing. In this last case, the same methods and type of means
would be used as in the corresponding case for the second stage
granular media filters, using controlled valve means 60 in a reject
pipe 61, as schematically illustrated in FIG. 1. Replacement water
can be supplied in the form of effluent from parallel filters being
in filtration mode, from second stage filters or from an external
source.
[0076] The present invention is not limited to the systems
described above but is suitable to use in applications of all kinds
where a high purity liquid is to be produced by filtration.
* * * * *